This invention relates to the field of recovering and purifying compounds, in particular the purification of acrolein or propionaldehyde, from a dilute aqueous solution containing impurities.
Acrolein and propionaldehyde are important industrial chemicals useful in a variety of organic syntheses. For example, acrolein may be reacted with methyl mercaptan to form beta-methylthiopropionaldehyde (MTPA).
Acrolein is conventionally produced by the vapor-phase oxidation of propylene over a solid-phase catalyst. This reaction produces a gaseous mixture containing acrolein, gases (e.g., nitrogen, oxygen, carbon monoxide, and carbon dioxide), propylene, water, and reaction by-products such as acrylic acid, acetic acid, formic acid, formaldehyde, acetaldehyde, allyl alcohol and polymers resulting from the degradation of acrolein.
Typically, acrolein is purified from the reaction effluent gases by absorption into water, resulting in a dilute aqueous acrolein stream (typically less than 5% acrolein) that also contains light-boiling reaction byproduct impurities such as formaldehyde, acetaldehyde, propionaldehyde, allyl alcohol and acetone. The crude, dilute aqueous acrolein produced in the absorption step is then subjected to distillation to separate the acrolein/water azeotrope. Present purification methods, which are well-known in the art, involve the multi-column distillation of the crude acrolein solution to recover relatively pure product. See, for example, U.S. Pat. No. 3,433,840 of Takesaburo et al.
A typical multi-column distillation for acrolein recovery has three columns, and operates as follows: A concentration column (1st column) separates concentrated crude acrolein as an overhead distillate product, with the water and heavier-boiling impurities going to the bottom of the column. The water may be recycled. The acrolein distillate (typically more than 92 wt % pure) is at or near its azeotropic water content, and contains light-boiling impurities such as acetaldehyde.
The acrolein distillate from the first column is then sent to a xe2x80x9clightsxe2x80x9d removal column (2nd column), which separates light-boiling impurities, especially acetaldehyde, as a distillate product. The lights-depleted concentrated acrolein then goes xe2x80x9cto the bottomsxe2x80x9d; i.e., falls to the bottom of the lights removal column for passage to the next column.
Note that acetaldehyde cannot be efficiently separated from the acrolein before passage through the concentration column, because acetaldehyde has too high an affinity for water. It is known that an acetaldehyde distillate isolated upstream of the concentration column contains significant amounts of acrolein, representing an undesirable acrolein recovery loss.
After passing through the lights-removal column, the lights-depleted acrolein concentrate (also called the xe2x80x9clights column bottomsxe2x80x9d) is sent to a product column (3rd column), where purified acrolein is taken as a distillate product, and heavy impurities and acrolein degradation products are removed from the column bottoms. A fraction of the acrolein is lost to the bottoms of the column, due both to limitations on the separation and the formation of thermal degradation products (e.g., acrolein dimer and polymer formation; see below).
Optionally, the product column may be operated as an extractive distillation, whereby an intermediate-boiling solvent is added to the column. The solvent goes to the bottom of the recovery column, diluting and reducing the boiling temperature of the lower portion of the column. The xe2x80x9csolvent/heaviesxe2x80x9d bottoms stream is sent to a solvent recovery column, where the solvent is taken as a distillate product and recycled to the purification column. The heavy-boiling impurities and thermal degradation products are removed via the bottoms. Depending on the solvent chosen, additional water removal (to below the normal acrolein azeotropic composition) may be achieved.
Although widely used, multi-column distillation methods for recovery of acrolein have number of significant cost and operational disadvantages.
For example, the multiple columns and supporting apparatus required for this process represent a major capital investment for a commercial acrolein production facility. It is also expensive to maintain a multiple distillation column system.
Moreover, each column in the multi-column system cannot achieve perfect separation, and thus there is a cumulative acrolein recovery loss of up to several percent due to separations losses in each distillation step.
Also, acrolein is a thermally-sensitive monomer which forms cyclic dimers and linear polymers on exposure to heat. These dimers and polymers are referred to herein as xe2x80x9cthermal degradation products.xe2x80x9d The amount of thermal degradation products formed in any system follows certain non-linear functions with respect to temperature and acrolein concentration/residence time in the distillation system (the function is exponential with respect to temperature; and xe2x80x9cpower-lawxe2x80x9d with respect to concentration/residence time). Thus, higher temperatures or extended residence times in a distillation system results in excessive production of thermal degradation products.
The thermal degradation products are insoluble, and may foul the internal workings and heat exchangers of distillation equipment. Excessive production of thermal degradation products during distillation necessitates frequent shut-down and cleaning of the distillation equipment, resulting in lost production and high maintenance costs. Furthermore, any thermal degradation products formed represent an unrecoverable acrolein yield loss, which translates into an economic penalty for the acrolein manufacturer.
During multi-column distillation, acrolein is repeatedly exposed to high temperatures and experiences relatively long residence times in the columns.
Thus, significant production of thermal degradation products occurs, resulting in unrecoverable yield losses and rapid fouling of the distillation system.
Production of thermal degradation products may be reduced by the addition of acrolein polymerization inhibitors (e.g., hydroquinone, phenothiazine, and derivatives of hydroquinone or phenothiazine) to the distillation system. As the thermal degradation products may form at any stage of the distillation process, polymerization inhibitors must be added to the top of each column in a multi-column distillation system. Moreover, unreacted polymerization inhibitors are lost to the heavy bottoms byproduct, and may ultimately contaminate the acrolein product. The use of polymerization inhibitors in a multi-column distillation system can therefore represent a significant operating cost for acrolein manufacturers.
Propionaldehyde has vapor-liquid equilibrium (VLE) and chemical properties nearly identical to acrolein. Purification of this compound by multi-column distillation has similar drawbacks as described above for acrolein.
What is needed, therefore, is a method of purifying acrolein or propionaldehyde which can be accomplished with less equipment (and thus less capital costs), which allows efficient product recovery without the separation losses inherent in multi-column systems, and which uses lower temperatures and residence times so as to reduce formation of thermal degradation products.